1. Field of the Invention
The present invention relates to an internal combustion engine with a dual turbocharger system having a first turbocharger and a second turbocharger in which the engine operation can be changed between a "one-turbocharger-operation," wherein only the first turbocharger is operated, and a "two-turbocharger-operation," wherein both the first turbocharger and the second turbocharger are operated.
2. Description of the Related Art
As illustrated in FIG. 8, a conventional turbocharged internal combustion engine 1' is provided with a first turbocharger 7' and a second turbocharger 8' having turbines 7a' and 8a', respectively, and compressors 7b' and 8b', respectively. An exhaust switching valve 17' is installed in an exhaust conduit 20b' downstream of the turbine 8a' Similarly, an intake switching valve 18' is installed in an intake conduit 14b' downstream of the compressor 8b'. Further, an intake bypass valve 33' is installed in an intake bypass conduit 13' which bypasses the compressor 8b'. The intake switching valve 18' and the exhaust switching valve 17' are closed at small to medium intake air quantities to produce a "one-turbocharger-operation" wherein only the first turbocharger 7' is operated. The intake switching valve 18' and the exhaust switching valve 17' are opened at large intake air quantities to produce a "two-turbocharger-operation" wherein both the first turbocharger 7' and the second turbocharger 8' are operated.
Japanese Patent Publication SHO 59-145328 and Japanese Patent Publication SHO 61-112734 disclose running-up the second turbocharger before the "one-turbocharger-operation" is changed to the "second-turbocharger-operation" for the purpose of suppressing torque shock by partially opening the exhaust switching valve or providing an exhaust bypass valve and opening the exhaust bypass valve.
However, there are problems with running-up of the second turbocharger.
More particularly, when the exhaust switching valve is partially opened or the exhaust bypass valve is opened, a portion of the exhaust gas begins to flow through the second turbocharger turbine and, as shown in FIG. 9, the amount of exhaust gas flowing through the first turbocharger turbine momentarily decreases. As a result, a momentary decrease in the turbocharger rotational speed occurs accompanied by a momentary decrease in the charging pressure and a momentary, relatively large torque shock when the second turbocharger begins to be run-up.
If on the other hand the amount by which the exhaust switching valve is opened is selected to be small for the purpose of suppressing the above-described torque shock, the running-up of the second turbocharger will be insufficient. As a result, as shown in FIG. 10, a large torque shock will take place when the exhaust switching valve is fully opened and the engine operation is changed from the "one-turbocharger-operation" to the "two-turbocharger-operation".